Accurate measurement of the flow of fluids in a conduit is of extreme importance in many industries. Many devices and processes have been developed over the years to try to obtain more accurate measurements of fluid flow. One method, which is widely known, is the pressure differential type in which a pressure differential produced as the fluid passes through a restriction in a conduit is used to calculate fluid flow. Another method is a pitot tube method.
Another known method, used to measure fluid flow, is generally referred to as a “thermal convection mass flow method”. There are two widely-known types of thermal mass flow meters, energy balance and convection. Energy balance flow meters generally use a small diameter capillary tube having a large length/diameter ratio to ensure fully developed fluid velocity and temperature profiles. The temperature of the fluid is measured at the inlet and outlet, and a constant source of heat is added to the fluid stream. The temperature gain is a function of the specific heat of the fluid and the mass flow, and follows the First Law of Thermodynamics. These devices are generally used for small clean gas flows with modest flow and temperature ranges.
In a second type of thermal mass flow meters, thermal convection mass flow meters, a heated sensor is inserted into a fluid stream. Since convective heat transfer is dependent on the fluid transport properties and the temperature difference between the heated sensor and the fluid, a temperature sensor is incorporated into the mass flow sensor and is used for temperature compensation. Two basic types of thermal convection mass flow sensor electronic circuits are in general industrial use with this type of meter: Constant Power Anemometer (CPA), and Constant Temperature Anemometer (CTA). A Constant Power Anemometer (CPA) provides constant electrical power to a resistance element. A temperature sensor is attached to the heater and is heated by conduction from the heater element. The difference between the temperature of the heated sensor and the ambient fluid temperature sensor is measured. The temperature difference is large at a low velocity and small at a high velocity. The temperature difference signal is conditioned to be linear with the mass velocity, and ambient temperature compensation is usually accomplished with analog signal processing. Constant power anemometers are slow to respond to changes in velocity and temperature because of the thermal inertia of the sensors; they do not have a stable “zero” because of the increased free convection caused by the high sensor temperature at zero flow; and unless specially corrected, they have a limited range of temperature compensation (±30° F.). Most CPA's use three sensors to provide the power, heated sensor temperature and ambient sensor temperature, and are very sensitive to non-axial velocity components because of the non-symmetric shape of the sensors.
The other basic type of thermal convection mass flow sensor electronic circuit is the Constant Temperature Anemometer (CTA). In this instrument, a single Resistance Temperature Detector (RTD) sensor is operated by a solid-state feedback control circuit to maintain a constant temperature difference between a heated sensor and the process fluid temperature, which is measured by a second Resistance Temperature Detector (RTD) sensor. The amount of electrical power needed to maintain this temperature difference is the measured output variable. As the fluid temperature changes, the CTA control circuit maintains a nearly constant “over-heat” temperature difference between the heated sensor and the ambient fluid temperature. The CTA circuit has a significant advantage over the CPA circuit because temperature compensation can be made for the temperature difference and the rate of change of the temperature difference. The CTA has several advantages over the CPA, these advantages are:
A high level output is obtained. In most cases, a power transfer ratio of 9:1 from zero velocity to 200 SFPS is obtained.
Only two sensors are needed, rather than three as used in a CPA.
CTA's have a much faster response to velocity changes than CPA's because only the outer surface of the heated element must be heated. Thus, CTA's have a velocity time constant of about 1 second and are 5 to 10 times faster than the time constant if used as a temperature sensor.
CTA'S, if properly designed, have a much smaller time constant than CPA's for fast changes in ambient fluid temperature.
CTA's are much less sensitive to the angle of velocity approach because the two sensors are circular and symmetrically mounted.
In order to obtain accurate measurements with a fluid flow measuring device, it is important to have an accurate and dependable sensor and related control circuit, such as described above, and to have that sensor located in the fluid stream at a location which is representative of the flow of the fluid. Ideally, having a fluid velocity profile which is flat would simplify obtaining such representative fluid flow; however such fluid velocity profile is not normally encountered, especially in sections of conduit upstream and downstream of pipe size changes, elbows, valves and other flow disturbances.